Automotive headlamps are commonly made with tungsten filaments. In one construction the coil ends are trapped between the folded ends of lead wires. A lead wire is a typically a nickel iron or molybdenum rod with a substantially greater diameter. The fold on the lead end is pressed closed to trap an end of the coil. If the fold presses directly on the coil, the turns of the coil are variably flexed, twisted, or turned, as they are crush in the press. This distorts the remaining portions of the coil.
Alternatively, a coil may be formed with a leg portion comprising a straight section of the coil wire that extends away from the coil body. The leg is then trapped in the lead. Generally this results in less coil distortion, but not completely so. The legs themselves may be bent, twisted or turned in being held by the lead. Since the coil wire is substantially thinner than the lead wire, the lead has to be distorted a great deal to accommodate the smaller coil leg wire. The leads are then pre-formed, flattened or similarly prepared for the final capture between the coil leg and the lead wire. Nonetheless variations in the lead preliminary or final formation, act to variably squirm the coil leg and therefore the coil itself.
An alternative to the direct coil leg to lead coupling is to provide an interface between the two wires. The lead then does not have to be bent so much during final deformation to capture the coil leg. Less distortion has then been achieved. The interface is typically a small sleeve that is slipped over an end of the coil leg and clamped in place. The sleeve provides a thicker leg for the coil, which is then easier to grasp by the lead. Attaching the sleeve has its own set of complications, costs and difficulties. In the end, it is still subject to transmitting the coupling distortions into the coil body.
Another difficulty with the coil and leg constructions is the leg itself is formed as part of the coil, which can leave an initial distortion in the coil.
Automotive filament coils are commonly made individually with the end sections formed as legs bent after winding the helix. Due to variations in wire weight, composition, winding temperature, and other factors the same number of mechanical turns in coil winding can bend the wire more (over wind) or less (under wind). The coils then have larger or smaller diameters and the legs are angularly more or less offset. Bringing the legs in to proper alignment by turning them as little as 25 degrees tensions to coil enough to “squirm” it. Bending the legs for subsequent attachment can also distort the coil. The coil has to be held while the bend is made. This tends to distort the coil with unpredictable results. The local crystal structure of the coil varies along its length, so the distortion may be relieved at any random point. The resulting coils are then bowed unpredictably. This is acceptable for Edison bulbs, but it is bad for fine optical applications such as headlamps.